Add classes for the category and tags to the primary <section>

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR adds a post's category and tag classes to the primary <section> tag of single posts. This lets us add styles for just posts in one category or tag.

These are already on the <article> tag (along with some of WordPress's other standard classes), but the way I structured the single posts means the title is outside of the <article>. Ideally this will be refactored in the near future, but this will help for the near term.

How to test the changes in this Pull Request:

1. Apply the PR.
2. View a post with both tags and categories; confirm they're being added as classes to the <section> tag with the primary ID, prefaced with category- or tag- -- something like:
   <section id="primary" class="content-area category-entertainment category-featured tag-testing-things tag-wondering">
3. Test the same with the One Column template.
4. Test the same with the One Column Wide template.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[claudiulodro]

Works great. Clever solution. 👍

[laurelfulford]

Thanks!